Structural, mechanical and electronic properties of two-dimensional chlorine-terminated transition metal carbides and...

Two-dimensional transition metal carbides and nitrides (MXenes) have attracted intensive attention since 2011, and surface groups have been determined to show a key role in MXene properties. Recently, an emerging functional group of chlorine was realized in MXenes, such as in Ti₃C₂Cl₂ and Ti₂CCl₂. In order to understand and apply MXenes terminated by this type functional group, the structural, mechanical and electronic properties of M₂X₂Cl₂ and M₃X₂Cl₂ (M  =  Sc, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W; X  =  C, N) are investigated in this work from first-principles theory. For M₂X₂Cl₂, the chlorine groups are all stabilized on the top-sites of the bottom M atoms on both sides. In M₃X₂Cl₂, the surface groups of most configurations are functionalized on the top-sites of the middle M atoms. Regarding to the mechanical properties, the elastic constants vary significantly with the types of M and X elements. The highest Young modulus of 208.3 GPa is determined in Ta₃C₂Cl₂. The mechanical, dynamical and thermodynamic stabilities are further tested. Thirteen members.ie. M₂CCl₂ (M  =  Sc, Ti, Zr, Nb, Hf), M₂NCl₂ (M  =  Sc, Zr), M₃C₂Cl₂ (M  =  Ti, Zr, Hf) and M₃N₂Cl₂ (M  =  Sc, Ti, Zr) are determined to be stable. In addition, Ta₂CCl₂, Hf₂NCl₂, M₃C₂Cl₂ (M  =  Sc, Nb, Ta) and Hf₃N₂Cl₂ could be metestable. For these stable and metastable configurations, the electronic structures and work functions are studied. Most configurations are metallic, except for Sc₂CCl₂, Zr₃N₂Cl₂ and Hf₃N₂Cl₂. These three members are indirect band gap semiconductors, with their band gap values of 1.65, 0.135 and 0.246 eV, respectively. All the structures show high work functions, and the smallest value is approximate to 3.97 eV determined in Ti₂CCl₂. Our work implies that the chlorine-functionalized MXenes could be utilized in semiconductors and metallic films.